Properties of Biodiesel and Other
Bi
Biogenic
i Fuels
F l
Gerhard Knothe
USDA / ARS / NCAUR
Peoria, IL 61604
U.S.A.
E-mail: [email protected]
1
Background
• Energy security
• Reduce dependence on petroleum-derived fuels
• Derived from renewable resources
• Enhance domestic (agricultural) economy
• “Green”
Green benefits: Biodegradability
Biodegradability, reduced exhaust emissions
2
Historical Background
Biofuels are not a recent concept.
• Considerable research and use in the early 1900’s.
• “Age of petroleum” approximately from 1945 to the 1970’s.
• Renewed interest in alternative fuels in the 1970’s due to energy
crises.
3
Historical Background
• Vegetable oil-based fuels
• First demonstration at the 1900 World Exposition
• Interest through the early 1940’s
• Late 1940’s / earlyy 1950’s some work at Ohio State U and
Georgia Institute of Technology
• First biodiesel described in 1937 Belgian Patent 422,877
• Ethanol
• Use and interest as blends / antiknock agent with gasoline
through the 1930’s.
• Tetraethyl lead as antiknock agent discovered in 1920’s
4
Petroleum-Based Fuels
Hydrocarbons:
• Gasoline: “Lighter” branched hydrocarbons
• Exemplified
p
byy 2,2,4-trimethylpentane
yp
(iso-octane)
(
)
• Diesel fuel: Long straight-chain hydrocarbons
• Exemplified by hexadecane (cetane)
• Jet fuel: “Light”
Light diesel fuel
5
Biogenic Fuels / Biofuels
• Most common biogenic
bi
i fuels
f l are oxygenated:
d
• Alcohols (ethanol, butanol): By fermentation of carbohydrates
• Gasoline replacement
• Fatty acid methyl (alkyl) esters: By transesterification of
triacylglycerol-containing materials
• Diesel fuel replacement
• Other biofuels include DME
DME, biomethanol,
biomethanol biohydtrogen
• More recently, hydrocarbon fuels from biofeedstocks (renewable
diesel, biogasoline) simulating petrofuels developed
6
Biodiesel and Renewable Diesel
Biodiesel
Methyl (alkyl) esters
By transesterification
Positive energy balance
Renewable Diesel
Hydrocarbons
By hydrodeoxygenation
Energy balance?
No/ low sulfur and aromatics
Regulated exhaust emissions reduced compared to conventional petrodiesel
Exception:
p
NOx
Cold flow problematic
Oxidative stability problematic
High lubricity
Biodegradable
Higher flash point
Cold flow varies /”adjustable”
“Lighter” form”: aviation fuel
High oxidative stability
Low lubricity
Biodegradability likely similar to
conventional petrodiesel
Flash point likely closer to petrodiesel
Feedstock availability and costs problematic
Well-researched
Other issues?
7
Fuel Suitability of Fatty Acid Methyl
(Alkyl) Esters
• Long-chain, unbranched alkanes are “ideal” petrodiesel fuels.
• At least in terms of combustion.
• Branched compounds and aromatics have low CNs
• Structural similarity of long-chain, unbranched alkanes shows
suitability of fatty esters as diesel fuels.
• Long-chain fatty compounds such as methyl palmitate and
methyl stearate have CN comparable to hexadecane
• Minimum CN in biodiesel standards > than in petrodiesel standards.
• 47 in ASTM D6751 (biodiesel) vs. 40 in ASTM D975 (petrodiesel).
• 51 in EN 14214 (biodiesel) vs. 51 in EN 590 (petrodiesel)
8
Why biodiesel and not the neat oil?
CH2-OOCR1
|
CH-OOCR2
|
CH2-OOCR3
Vegetable Oil
(Triacylglycerol)
R‫׳‬OOCR1
Catalyst
+
3 R‫׳‬OH
→
R‫׳‬OOCR2
+
R‫׳‬OOCR3
Alcohol
Vegetable Oil Alkyl Esters
(Biodiesel)
CH2OH
|
CHOH
|
CH2OH
Glycerol
Viscosity!
27-35 mm2/sec
4-5 mm2/sec
Kinematic viscosity of petrodiesel fuels usually ≈ 1.8-3.0 mm2/sec.
9
Biodiesel and Alcohols
• Biodiesel
• Properties can vary considerably depending on the feedstock
• Feedstocks can have widely varying fatty acid profiles
• “Conventional” oils, i.e. classical commodity oils and many
“alternative” oils contain mainlyy p
palmitic,, stearic,, oleic,, linoleic
and linolenic acids.
• Other oils may contain shorter or longer chain acids
• Many algal oils contain highly polyunsaturated fatty acids.
• Alcohol fuels
• Same product independent of the feedstock
10
Some Fuel and Physical Properties
of Ethanol and Butanol
Ethanol
Octane number (RON/MON*)
Melting
g point
p
(°C)
( )
Boiling point (°C)
Heating value [MJ/kg], [MJ/l]
107-112 / 88-90
-114
78.3
29, 23
Butanol
92-96 / 71-79
-88.6
117.7
37, 30
* RON = research octane number, MON = motor octane number
• Suitability of alcohols as fuel (component) demonstrated by high octane
number similar to cetane number of biodiesel.
11
Biodiesel Standard ASTM D6751
Property
Test method
Flash point (closed cup)
D 93
Alcohol control.
control One of the following must be met:
1. Methanol content
EN 14110
2. Flash point
D 93
Water and sediment
D 2709
D 445
Kinematic viscosity, 40 oC
Sulfated ash
D 874
Sulfur
D5453
Copper strip corrosion
D 130
Cetane number
D 613
Cloud point
D 2500
Carbon residue
D 4530
Acid number
D 664
Free glycerin
D 6584
Total glycerin
D 6584
Phosphorus content
D 4951
Distillation temperature,
D 1160
Atmospheric equivalent temperature, 90% recovered
Sodium and potassium, combined
EN 14538
Calcium and magnesium, comb.
EN 14538
Oxidation stability
EN 15751
Cold soak filterability
D7501
Limits
93 min
Units
oC
0.2 max
130 min
0.050 max
1.9-6.0
0.020 max
0.05 or 0.0015 max a)
No. 3 max
47 min
Report
0.050 max
0.50 max
0.020
0.240
0.001 max
360 max
% volume
130 min
% volume
mm2 / s
% mass
% mass
5 max
5 max
3 min
360 max
oC
% mass
mg KOH / g
% mass
% mass
% mass
oC
ppm (μg/g)
ppm (μg/g)
hours
sec
a) The limits are for Grade S15 and Grade S500 biodiesel, respectively. S15 and S500 refer to
maximum sulfur specifications (ppm).
12
Biodiesel Standard EN 14214
Property
Ester content
Density; 15oC
Viscosity 40 oC
Viscosity,
Flash point
Sulfur content
Carbon residue (10% dist. res.)
Cetane number
Sulfated ash
Water content
Total contamination
Copper strip corrosion (3h,
(3h 50oC)
Oxidative stability, 110 oC
Acid value
Iodine value
Linolenic acid content
Content of FAME with ≥ 4 double bonds
Methanol content
Monoglyceride content
Diglyceride content
Triglyceride content
Free glycerine
Total glycerine
Alkali metals (Na + K)
Earth alkali metals (Ca + Mg)
Phosphorus content
Test method
EN 14103
EN ISO 3675, 12185
EN ISO 3104,
3104 ISO 3105
EN ISO 2719, 3679
EN ISO 20846, 20884
EN ISO 10370
EN ISO 5165
ISO 3987
EN ISO 12937
EN 12662
EN ISO 2160
EN 14112, 15751
EN 14104
EN 14111
EN 14103
Limits
96.5 min
860-900
35 50
3.5-5.0
101 min
10.0 max
0.30 max
51 min
0.02 max
500 max
24 max
1
8.0 min
0.50 max
120 max
12 max
1 max
EN 14110
0.20 max
EN 14105
0.80 max
EN 14105
0 20 max
0.20
EN 14105
0.20 max
EN 14105, 14106
0.02 max
EN 14105
0.25max
EN 14108, 14109, 14538 5.0 max
prEN 14538
5.0 max
EN 14107
4.0 max
Units
% (m/m)
kg/m3
mm2/s
oC
mg/kg
% (m/m)
% (m/m)
mg/kg
mg/kg
h
mg KOH / g
g iodine /100g
%(m/m)
% (m/m)
% (m/m)
% (m/m)
% (m/m)
%(m/m)
%(m/m)
%(m/m)
mg/kg
mg/kg
mg/kg
13
Major Ester Components of Most
Biodiesel Fuels
Fatty esters in from common vegetable oils (palm, soybean,
canola/rapeseed, sunflower, etc):
•
•
•
•
Methyl palmitate (C16:0): CH3OOC-(CH2)14-CH3
Methyl stearate (C18:0): CH3OOC-(CH2)16-CH3
Methyl
y oleate ((C18:1, Δ9c)): CH3OOC-(CH
( 2)7-CH=CH-(CH
( 2)7-CH3
Methyl linoleate (C18:2; all cis):
CH3OOC-(CH2)7-(CH=CH-CH2)2-(CH2)3-CH3
• Methyl linolenate (C18:3; all cis):
CH3OOC-(CH2)7-(CH=CH-CH2-)3-CH3
From other oils:
• Methyl laurate (C12:0): CH3OOC-(CH2)10-CH3
• Methyl ricinoleate (C18:1, 12-OH; cis):
CH3OOC-(CH2)7-CH=CH-CH2-CHOH-(CH2)5-CH3
14
Properties of Methyl Esters
Cetane
M.P. Kin. Visc.
C12:0
C16:0
C18:0
Number
67
85
100
(°C) (40°C; mm2/s) Stab. (h)
4.5
2.43
> 24
28.5
4.38
> 24
38
5.85
> 24
Comb. (kJ/kg)
37968
39449
40099
C18:1
C18:2
C18:3
58
38
23
-20
-43
-52
4.51
3.65
3.14
2.79
0.94
0
40092
39698
39342
C18:1 12-OH 37
-5
15.29
0.67
ASTM 6751 47 min
EN 14214 51 min
CP
CFPP
1.9-6.0
3.5-5.0
Oxid.
3 min
8 min
Heat of
-
G. Knothe; Energy & Fuels 22, 1358-1364 (2008).
15
Some Fatty Acid Profiles
Vegetable Oil
C16:0
C18:0
C18:1
C18:2
C18:3
Rapeseed / Canola
3-4
1-3
58-62
20-22
9-12
Soy
8-13
2-6
18-30
49-57
2-10
Sunflower
6-7
3-5
21-29
58-67
Jatropha
13-15
7-8
34-44
31-43
16
Properties of Vegetable Oil Esters
Methyl Ester
Rapeseed / Canola
Cloud Point
(°C)
-3
Cetane Number
52-55
Kin. Visc.
(40°C; mm2/s)
4.5
Soy
0
48-52
4.1
Sunflower
0
≈ 55
4.4
Jatropha
4-5
Oxidative stability: usually antioxidants required to meet standard
specifications
17
Property Trade-off
Increasing chain length:
• Higher melting point (-)
• Higher
g
cetane number ((+))
Increasing unsaturation:
• Lower melting point (+)
• Decreasing oxidative stability (-)
• Lower cetane number (-)
18
Influence of Alcohol Moiety
Branched and longer-chain
longer chain esters:
● Lower melting points, similar cetane numbers compared to methyl esters
Ester
C16:0
C16:0
C16:0
C16:0
Methyl
Ethyl
Propyl
iso-Propyl
C18:1
C18:1
C18:1
C18:1
Methyl
Ethyl
Propyl
iso-Propyl
M.P. (°C)
28.5
23.2
20.3
13-14
CN
85.9
93.1
85.0
82.6
Ester
C18:0
C18:0
C18:0
C18:0
-20.2
-20.3
-30
30.5
5
59.3
67.8
58 8
58.8
86.6
C18:2 Me
C18:2 Et
C18:2 Pr
Me
Et
Pr
i-Pr
M.P. (°C)
37.7
33.0
28.1
-43.1
-56.7
CN
101
97.7
90.0
96.5
38.2
39.6
44 0
44.0
● Disadvantage: Higher costs of alcohols
Source: Handbook of Chemistry and Physics; The Lipid Handbook, various publications.
19
Five Approaches to Improving
Biodiesel Fuel Properties
Unchanged fatty
ester composition
Additives
A
Change
alcohol
B
Physical procedures C
Modified fatty ester
composition
Change fatty acid
profile
Genetic
D
modification
Inherently different
fatty acid profile
Alternative
feedstocks
E
G. Knothe; Energy & Environmental Science, 2, 759-766 (2009).
20
Exhaust Emissions Studies
Average effect of biodiesel and B20 vs.
vs
petrodiesel on regulated emissions
(Source: USEPA report 420-P-02-001)
Petrodiesel
B20
Petrodiesel
Biodiesel
100
Relative Emissions
Rellative emissions
100
80
60
40
20
80
60
40
20
0
0
NOx
PM
CO
Pollutant
HC
NOx
PM
CO
HC
Pollutant
21
NOx and PM Exhaust Emissions of
Petrodiesel, Biodiesel, Their Components
2003 Engine; EPA Heavy Duty Test
2.5
2.0
NOx
PMx10
1.5
1.0
2007PM Standard
0.5
te
ra
e
au
as
hy
et
et
M
M
hy
D
lP
lL
m
al
B
i ta
an
ec
od
2
te
e
e
an
ec
ad
ex
H
et
M
So
y
B
hy
io
lO
di
le
a
es
as
e
el
te
0.0
B
Brake-Specific Emission
n Rate, g/hp-hr
3.0
G. Knothe, C.A Sharp, T.W. Ryan III, Energy & Fuels 20, 403-408 (2006).
22
Change in Exha
aust Emissions Relative to Base Fuel (%)
-20
-30
-40
Me oleate
Me soyate
Dodecane
Me laurate
Me palmitate
e
-50
e
Hexadecane
Change in NOx and PM vs. petrodiesel
NOx
PM
10
0
-10
-60
-70
-80
23
Biodiesel and Lubricity
• Neat biodiesel has excellent lubricity as do neat methyl esters.
• Not included in biodiesel standards.
Low-level
level blends ((~ 2% biodiesel in petrodiesel = B2):
• Low
• Lubricity benefits through biodiesel with (ultra-)low sulfur
petrodiesel fuels which do not possess inherent lubricity
compared to non-desulfurized petrodiesel fuels.
• Marginal cost impact.
• High-frequency reciprocating rig (HFRR) tester (ASTM D6079; ISO
12156) in ASTM and EN petrodiesel standards.
• Maximum wear scars of 520 (ASTM) and 460 μm (EN).
24
Biodiesel and Lubricity
Lubricity of low-level blends of biodiesel with petrodiesel to a great extent
dete mined by:
determined
b
•
Contaminants (minor components), especially free fatty acids and
monoacylglycerols.
• In the neat form, even better lubricity than methyl esters.
• Disproportionately affect lubricity of low-level blends.
• Glycerol
y
has limited effect ((insolubilityy in p
petrodiesel).
)
Example (HFRR wear scars):
•
ULSD: 651, 636 μm
• w. 1% methyl oleate: 597, 515 μm
• 1% oleic acid in methyl oleate, then 1% thereof in
ULSD: 356,
356 344 μm.
μm
• w. 2% methyl oleate: 384, 368 μm
G. Knothe, K.R. Steidley; Energy & Fuels 19, 1192-1200 (2005).
25
Water in Biofuels
• Oxygenated nature of biofuels causes some differences in properties
compared to hydrocarbons as well as some similarities between the
biofuels.
• Oxygen content causes some affinity for water:
• Biodiesel can contain 1000-1700 ppm water, petrodiesel 40-114
under same conditions (4-35ºC; 300 h equilibration time)
(He et al, Appl. Eng. Agric, 2007).
• Limited in biodiesel standards to 500 ppm.
• Ethanol is hygroscopic.
h g oscopic Limited in ASTM D4806 to 1%.
1%
• Water in biofuels can promote fuel degradation with acid formation.
26
Ethanol
Ethanol: Formation of acetic acid (pKa = 4.76) with Acetobacter
• ASTM D4806
• Limited to 0.007 mass % (by ASTM D1613);
• pHe
H 6
6.5-9.0
5 9 0 (by
(b ASTM 6423;
6423 for
f fuels
f l containing
t i i nominally
i ll
≥ 70 vol % ethanol)
• ASTM D7795: Acidity in Ethanol and Ethanol Blends by Titration;
• For low levels of acidity:< 200 mg/kg.
• ASTM D7577: Accelerated Iron Corrosion Rating of Denatured Fuel
Ethanol and Ethanol Blends.
,
27
Biodiesel
• Formation of free fatty acids through hydrolysis.
• Water can promote microbial growth:
• Can affect marine applications
pp
as seawater is used as
ballast in tanks when fuel consumed.
• Free fatty acids addressed by acid number in biodiesel standards
of max 0.50 mg KOH / g.
28
Biodiesel Oxidation
• Biodiesel affected by presence of oxygen in air.
• Unsaturated fatty acid chains susceptible to oxidative
degradation.
g
• Acids can be formed by oxidative degradation.
• Can be monitored by acid value and / or kinematic viscosity.
29
Properties of Fatty Acids vs. Fatty
Acid Methyl Esters
Oleic acid
Melting point (°C)
12.8
Kinematic viscosityy ((40°C; mm2/s))
19.91
Lubricity (HFRR test; μm; two tests)
0, 0
Methyl oleate
-20.2
4.51
290, 342
pKa of linoleic acid: 7.6 (Handbook of Chemistry and Physics).
• Free fatty acids also have a benefit: Improve lubricity.
lubricity
30
Summary
• Biogenic fuels (biofuels) have a greater affinity for water due to
their structure (oxygen content).
• Can promote the formation of acids.
• For biodiesel, acids also possible by oxidative degradation
• Acids with biodiesel weaker than with ethanol; free fatty acids
provide lubricity benefits.
• Biodiesel properties strongly dependent on fatty acid profile.
31
Parting Thoughts:
Rudolf Diesel (1912)
“The fact that fat oils from vegetable sources can be
used may seem insignificant to-day, but such oils may
perhaps become in course of time of the same
impo tance as some nat
importance
natural
al mine
mineral
al oils and the tar
ta
products are now. ... In any case, they make it
certain that motor-power can still be produced from
the heat of the sun, which is always available for
agricultural purposes, even when all our natural
stores of solid and liquid fuels are exhausted.”
exhausted ”
R. Diesel, The Diesel Oil-Engine, Engineering 93:395–406 (1912). Chem. Abstr. 6:1984
(1912).
32